The present invention relates to a device for thermally fixing a toner image on a copy sheet, the copy sheet being first heated to a temperature sufficient to partially fix the toner image, and then the copy sheet being contacted by a heat roller which functions to completely fix the toner image. More particularly, the present invention relates to the particular material which makes up the surface of the heat roller.
In the art, devices are known for thermally fixing of toner images, including devices which employ heat rollers for pressing and heating of copy sheets. Many attempts have been made to improve these types of devices. One of these attempts is represented in Japanese Laid Open Utility Model Publication No. Sho 47-22939 which was published on Nov. 15, 1972. In this device the copy sheet is heated in a preliminary heating step up to a temperature at which the toner image on the copy sheet becomes semi-fixed and thereafter the toner image is completely fixed by means of a heat roller. This device is effective in eliminating the drawbacks of other prior art fixing devices wherein the toner image is fixed by a heat roller alone. In such devices which only utilize a heat roller for toner image fixing, the heating period is quite short, and in order to enhance the efficiency of heat transfer from the heat roller it is necessary to utilize an increased pressing force to the heat roller and to enlarge the nip width so as to increase the amount of heat applied to the toner image. Typically, such pressures required have been as high as 10 to 50 Kg. Such pressures, however, resulted in damage to the heat roller, jamming or sticking of the copy sheet to the heat roller, etc. In contrast to this, the above-noted Japanese Utility Model Application allowed for a reduction in the pressing force required of the heat roller as well as a reduction in the offset of toner image and the jamming or sticking of the copy sheet to the heat roller.
However, the device of the above-noted Japanese Utility Model Application suffers from drawbacks of its own: after the fixing of several thousand copy sheets the material of the heat roller tends to change in quality, thereby resulting in the occurrence of offset of the toner image to the heat roller or the jamming and undesirable sticking of the copy sheets to the heat roller. To deal with this problem, frequent replacement of the heat roller must be resorted to, or else the application of silicone oil to the heat roller is required. Such countermeasures are, however, impractical because the former makes proper maintenance of the copy machine itself difficult and the latter requires the provision of a silicone oil application mechanism, thus resulting in a greater complexity of the construction of the device.
It has been proposed to omit the use of a silicone oil application mechanism and to instead use a heat roller impregnated with silicone oil, e.g., by soaking the heat roller in a bath of silicone oil for a long period of time. However, in situations wherein the toner image on the copy sheet is not subjected to a preliminary fixing, an increase in the amount of silicone oil impregnated in the heat roller will cause the heat roller to deform as a result of the necessarily applied high pressure. Thus, the amount of silicone oil contained in the heat roller is usually less than 20% by weight.
In U.S. Pat. No. 3,731,358 to Artl, it is taught that offset can be prevented when fixing toner images on copy sheets when the heat roller contains a silicone rubber outer layer. Furthermore, it is taught to impregnate the roller with a silicone oil to help avoid offset. However, no mention is made of any specific amounts of impregnated silicone oil, i.e., the amounts which would be necessary to prevent jamming or undesirable sticking of the copy sheet around the heat roller.
It is an object of the present invention to provide a heat roller for a thermal fixing device wherein excellent fixing of toner images on copy sheets can be obtained even after a large number of copy sheets have been subjected to the fixing treatment and wherein offset of the toner image to the surface of the heat roller and jamming or undesirable sticking of the copy sheets to the heat roller can be prevented.